OCT is an imaging modality that can be thought of as the optical analog to ultrasound. Focused light illuminates a sample and backscattered light is collected and by use of interferometry, depth gated as to where it backscattered from. This allows one to see into samples in a similar fashion to ultrasound. To build up a 2 or 3 dimensional image, the light beam is typically scanned across the sample in one or two directions. OCT is ideally positioned for imaging samples where 1 to 5 millimeters of penetration is needed with resolution of 2 to 15 microns.
OCT is widely used in an increasing number of applications including, but not limited to, medical (e.g., ophthalmology, intravascular, oncology, dermatology, neurology, gastroenterology, ear, nose and throat (ENT)), biomedical research (developmental biology, small animal imaging, biofilm imaging, and tissue engineering), and industrial (e.g., carbon fiber composites, art inspection, multilayer thin film thickness, plastic seal monitoring, contact lens production, and coating qualification). Lowering the system price will both increase the usage in these areas and open up new areas for application.
The first OCT systems used a time domain architecture where depth scanning was achieved by physically moving a mirror in the reference arm. In early 2000s, the Fourier domain approach to OCT was invented with two implementations, spectral domain OCT where a broadband light source is used in conjunction with a spectrometer and swept source OCT where a laser is swept in wavelength and different wavelengths are sampled at different times. Fourier domain OCT architectures have a SNR advantage over time domain ones that is proportional to the number of pixels in the spectrometer or the number of samples in one laser sweep. FD-OCT systems have now displaced time domain systems in most clinical applications although there are a few industrial applications where time domain OCT is still prevalent.
In spectral domain OCT system architecture there are four critical components that set the performance: the light source, the grating and camera inside the spectrometer, and the scanner. Even though OCT has commercially exploded in the last decade, it is not yet a large enough market by itself to drive component development. Therefore, the advanced components used in most OCT systems were originally developed for other applications. For example, most cameras used in OCT spectrometers are line scan cameras designed for machine vision applications. These cameras have very high line rates (up to 140,000 lines/second), but have short pixel dimensions since they are used to image items passing by quickly on conveyor belts such that the translation of the object provides the 2nd dimension for imaging. When used in spectrometers, these cameras are difficult to align and maintain since spectrally dispersed light forms a line that is approximately 20 mm wide by 6-7 microns tall and the line scan array is 20 millimeters wide by 20 microns tall. These cameras are also fairly expensive with even low end models costing at least $2,000.
Moving beyond the research and industrial markets, there is tremendous opportunity for low cost OCT in clinical areas, such as at the point of care and for clinical care in the developing world, but the regulatory and manufacturing requirements for a clinical system require more capital to address. Some potential clinical applications include a low cost retinal scanner that could be widely deployed both in the United States for imaging of patients with diabetic retinopathy, glaucoma, or macular degeneration or use in the developing world for retinal screening of newborns and infants. Regulatory overhead and more sophisticated software may increase the cost of a clinical unit, but it could still be greater than $12,000.
For at least the aforementioned reasons, there is a continuing need for low cost OCT imaging systems and techniques that provide high quality images.